In the 1930s, U.S. Navy researchers stumbled upon the concept of radar when they noticed that a plane flying past a radio tower reflected radio waves. Scientists have now applied that same principle to make the first device that tracks existing Wi-Fi signals to spy on people through walls.

Wi-Fi radio signals are found in 61 percent of homes in the U.S. and 25 percent worldwide, so Karl Woodbridge and Kevin Chetty, researchers at University College London, designed their detector to use these ubiquitous signals. When a radio wave reflects off a moving object, its frequency changes—a phenomenon called the Doppler effect. Their radar prototype identifies frequency changes to detect moving objects. It’s about the size of a suitcase and contains a radio receiver composed of two antennas ­and a signal-processing unit. In tests, they have used it to determine a person’s location, speed and direction—even through a one-foot-thick brick wall. Because the device itself doesn’t emit any radio waves, it can’t be detected.